Gears are usually made by performing a cutting work on a gear material, where the gear teeth are cut and formed by using a gear hobbing machine, or by performing a roll forming, where a round die or a flat die is pushed against the gear material to form the gear teeth. The cutting or forming process may include an error of a shape of a working tool, an operating error of a machining tool, an error of an installation position of the gear material or changes in cutting or forming environment. As a result, a geometric shape of the finished gear may include some errors. This may cause an error of a tooth thickness or runout of a tooth space of the gear, which may cause a backlash between mating gears or a decreased transmission efficiency, resulting in a short life of the gear. Therefore, it is important to inspect a tooth profile of the finished gear when, in particular, a high precision is required.
According to a known technique for inspecting gears disclosed in JPH5-209703, a pair of gauge heads is provided in a manner that a space between the gauge heads is increased or decreased by means of a driving device. For measuring a tooth thickness, a gear to be tested is placed so that plural teeth of the gear are sandwiched by the gauge heads at a time. According to the measuring device employing the technique disclosed in JPH5-209703, various kinds of gears each having a different specification are measured by using an identical measuring device. Specifically, the pair of gauge heads formed so as to have ridge-like lines arranged parallel to each other is arranged in a manner that the space between the gauge heads is increased or decreased. The pair of gauge heads makes contact with the plural teeth so that the plural teeth are sandwiched by the gauge heads at a time. A distance, which is perpendicular to the axis, across several teeth sandwiched by the gauge heads, that is, a sector span, is obtained by measuring a distance between the two gauge heads. By using this measuring device, the geometric shape of the gear is measured without using a master gear or the like, which spares a set-up time of the master gear. As a result, measurement efficiency is improved (refer to paragraphs 0023 to 0025, and FIG. 1 of JPH5-209703).
According to the known technique for inspecting gears disclosed in JPH5-209703, various kinds of gears each having a different specification are measured without using the master gear or the like. However, the gauge heads are made contact with particular two teeth among the teeth of the gear to be tested. Errors related to the gear, including an error of the tooth thickness, are not identical along a circumference of the gear. Therefore, the geometric shape of the finished entire gear is not determined even if the geometric shape of a particular portion of the gear is measured. In order to measure the geometric shapes of all the individual teeth of the gear by using the measuring device according to JPH5-209703, the measuring work may be complicated. In addition, the measuring device according to JPH5-209703 provides an approximate tooth thickness by measuring the distance between the two teeth, however, it may not be determined whether or not the entire gear is made into an intended shape. In case that the gear to be tested has a small-module, a small pressure angle or a large number of teeth, the measuring work may be even more difficult. As mentioned above, the gear shape measuring apparatus disclosed in JPH5-209703 may involve the complicated measuring work for grasping the geometric shape of the entire gear and there might be room for improvement.
A need thus exists for a gear shape measuring apparatus which is not susceptible to the drawback mentioned above.